Systems for delivering ionized drugs through the skin have been known for many years. The application of an electrical field to the skin has been found to greatly enhance the skin's permeability to various ionic agents. In general, iontophoretic methods include the introduction of various ions into tissues through the skin by means of electricity. Additionally, applying an electrical current to the area around a wound has been found to improve wound healing. Iontophoretic techniques have been used to deliver some drugs, thus eliminating the requirement for hypodermically injecting these drugs. The elimination of the requirement for hypodermic injection can reduce or eliminate the associated problems of pain, trauma, and infection to the patient.
The manufacture and use of conventional iontophoretic patches is well known to those skilled in the art. In particular, Flower discusses iontophoretic controllers, patches, and electrodes in U.S. Pat. Nos. 5,668,231, 5,830,175, 5,876,368, 6,009,344, 6,047,208, and 6,018,680. Manufacture of patches is discussed by Green et al. in U.S. Pat. No. 5,682,726. Electrodes and patch discussions are included in U.S. Pat. No. 5,651,768 (Sibalis), U.S. Pat. No. 6,038,464 (Axelgaard et al.), and U.S. Pat. No. 5,458,569 (Kirk, III et al.). Iontophoresis, including polymeric carriers, delivery vehicles, and medicants, are discussed by Keith in U.S. Pat. No. 4,542,013, by D'Angelo et al. in U.S. Pat. No. 6,024,975, and by Powers et al. in U.S. Pat. No. 4,702,732. All of the aforementioned patents, which include subject matter in addition to that specifically cited above, are herein incorporated by reference in their entirety.
Iontophoresis is an active method for transdermal delivery of medicants. Active delivery methods may be distinguished from passive delivery methods. Passive methods rely on natural forces and pressures alone such as diffusion or concentration gradients. Conventional drug delivery skin patches are examples of passive delivery vehicles. Active methods use externally applied forces, for example, electrical potential or hydraulic forces, to force a medicant into the skin. Iontophoretic patches are active delivery vehicles for medicants.
Prior art iontophoretic analgesic or anesthetic devices have been limited in use to broad area application of an anesthetizing agent, for example, an anesthetic gel. One drawback of existing devices is that they must be applied directly over the target area. This has been undesirable as the target area cannot be viewed during application of the anesthetic, and the patch must be removed to view the target area. Once the patch is removed to access the site, delivery of the anesthetic stops. This creates a likelihood that the anesthesia will wear off as the procedure progresses, causing increased pain. In order to continue, the treating physician must either reapply the patch and wait for it to take effect, inject the site with local anesthetic, continue the procedure in spite of the increasing pain, or convert to general anesthesia.
Another problem with prior art devices, where the target area includes a wound, is that the anesthetic gel is placed directly on the target area. This creates a possibility that the anesthetic gel will contaminate the wound, cause infection or inflammation, or hinder the healing of the wound. One prior art method avoids this problem by using a field block. Local anesthesia may be achieved by making several injections in the area surrounding the target site, thus creating what is commonly referred to as a field block. A field block procedure may be fairly painful in itself, and may traumatize the patient, particularly a child.
After removal of prior art delivery patches, or after completion of the field block, there is no fail-safe method for determining where the anesthetized area begins or ends. This creates the potential for continuing a procedure into an un-anesthetized area, causing pain to the patient.
What would be desirable is a medical device capable performing local anesthesia without many of the aforementioned limitations.